1. Technical Field
The present invention relates to an ultraviolet-curable ink composition for ink jet and an ink jet recording method.
2. Related Art
Ink jet recording methods, which allow the user to record high-definition images with relatively simple equipment, have been rapidly advancing in various fields. For example, JP-A-2012-188612 discloses an ultraviolet-curable ink composition for ink jet to provide an ultraviolet-curable ink composition for ink jet that has excellent curability and provides excellent abrasion resistance and stretchability to coatings cured therefrom. This ink composition contains a monofunctional (meth)acrylate monomer and a photopolymerization initiator. The molecular structure and the content of the monofunctional (meth)acrylate monomer and the Tg of the cured form of the monofunctional (meth)acrylate averaged by the number of moles are specified.
Furthermore, JP-A-2012-140583 discloses an ultraviolet-curable ink composition for ink jet to provide an ultraviolet-curable ink composition for ink jet that has excellent curability and prevents coatings cured therefrom from being colored soon after ink jet recording. This ink composition contains a selected monomer and a photopolymerization initiator that contains an acylphosphine-oxide-based photopolymerization initiator and a thioxanthone-based polymerization initiator. The total content of the acylphosphine-oxide-based photopolymerization initiator and the thioxanthone-based photopolymerization initiator is from 8% to 16% by mass based on the total mass of the ink composition.
This indicates that it has hitherto been believed that the use of a thioxanthone-based photopolymerization initiator, which is unlikely to be affected by oxygen inhibition, is an effective way to ensure the curability of an ink composition applied to a substrate. Research by the inventors, however, found that when a thioxanthone-based photopolymerization initiator is used, the curability of the ink composition is admittedly good near the surface of the ink composition but is not necessarily good in the ink composition.
More specifically, research by the inventors found that the use of a thioxanthone-based photopolymerization initiator, which exhibits a high percentage of light absorption at wavelengths around 395 nm, causes the ink composition to be lacking in curability inside, particularly in light curing with a semiconductor light source, such as an LED, as a result of inhibited transmission of light into the ink composition applied to a substrate. The research also revealed that this reduced internal curability leads to poor adhesion of the ink composition to the substrate. An ink composition that contains such a thioxanthone-based photopolymerization initiator and yellow coloring material and/or black coloring material, which both exhibit a low percentage of light absorption at wavelengths around 395 nm, in such a manner that coatings cured from the ink composition will have a predetermined OD (optical density) is less adhesive because of more severely inhibited inward transmission of light. An ultraviolet-curable ink composition for ink jet that contains yellow coloring material and/or black coloring material and exhibits excellent adhesiveness even when ultraviolet radiation with a peak wavelength in the range of 380 to 405 nm is used would become increasingly useful because UV ink jet printers, which use a low-cost semiconductor light source, such as an LED, to cure an ink composition, will be in demand.